1. Field of the Invention
Aspects of the present invention relate to an electrolyte for a lithium ion secondary battery and a lithium ion secondary battery comprising the same. More particularly, aspects of the present invention relate to an electrolyte that can improve capacity retention property, thermal stability and durability of a lithium ion secondary battery, even when the battery is left at a high temperature, by containing difluoro oxalato borate (DFOB) and fluoro ethylene carbonate (FEC) as additives, and a lithium ion secondary battery comprising the same.
2. Description of the Related Art
A battery is a device that converts the chemical energy of chemical materials inside the battery into electrical energy through an electrochemical oxidation/reduction reaction. Recently, portable devices such as camcorders, cellular phones, notebook computers, PCs and PDAs have been actively developed with rapid progress of the electronic, telecommunication and computer industries. Accordingly, there has been an increased demand for a slim secondary battery of high performance, durability and reliability that can be used in the above portable devices.
A lithium ion battery has been widely used as a secondary battery because it has a high discharge voltage near 4V, an excellent energy density per weight and a low self-discharge rate.
In the lithium secondary battery, the ion conductivity of the electrolyte greatly affects the charge/discharge performance of the battery. Thus, it is desirable that an electrolyte have a high ion conductivity. Accordingly, in the battery industry, many experiments have been performed to improve electrochemical characteristics such as the ion conductivity of the battery by mixing a solvent of a high dielectric constant and a solvent of a low viscosity. In addition, research has been actively conducted to improve the thermal stability of the battery by mixing a solvent of a high boiling point (Japanese Patent publication No. 1999-111306).
During initial charging of the lithium battery, lithium ions released from a positive electrode move to a carbon electrode used as a negative electrode and form a film on the surface of the negative electrode. Such a film is called as a SEI (solid electrolyte interface) film. The SEI film greatly affects on discharge capacity during subsequent cycles. Physical and chemical properties of the SEI film are changed according to salt used in the electrolyte, the concentration of the salt, components of solvent mixture, composition of the solvent mixture and the kind of additive used. When an uneven SEI film is formed, because an additive is not used or because undesirable additive is used, electrons in active materials are released and cause decomposition of the electrolyte. Accordingly, the irreversible capacity of the active material is increased and the capacity and lifetime of the battery is reduced. With respect to the SEI film, there has been much research directed to improving the physical and chemical properties of the film by changing the SEI film formation reaction by adding an additive to the electrolyte. For example, there have been disclosed techniques of adding CO2 to the electrolyte (Japanese Patent publication No. 1995-176323A) or preventing decomposition of the electrolyte by adding a sulfide group compound to the electrolyte (Japanese Patent publication No. 1995-320779A).
Accordingly, an electrolyte having high ion conductivity, high dielectric constant and low viscosity is desirable in the lithium secondary battery. In addition, a solvent capable of reducing decomposition and vaporization of the electrolyte after discharge is desirable to improve discharge characteristics of the battery at a high temperature. In addition, an additive is desirable to form an even SEI film that transfers lithium ions well but does not transfer other material.